Jewels are used to boost building time completion, hire builders, gamble with mugs, compete in lords league extra times a day, reassign your lords league challenges, revive heroes, but most importantly hire heroes. Easy ways to obtain gems would be to of course buy them but if you are free to play or just looking for a few extra gems they have free gem offers you can do by downloading apps, completing surveys, watching videos etc. You can also obtain jewels by doing dungeons and another way not many people know about is downloading the Google rewards app and they give you a survey every so often that takes a few seconds and you can use that as Google credit to buy jewels.